In the medical field there is a continuing need for new compounds having demonstrated use for inducing anesthesia. It is not only important to induce beneficial anesthesia, but it must be done in a manner that limits toxicity to patients, and as well, minimizes what is known as xe2x80x9canesthesia hangoverxe2x80x9d.
The pineal hormone melatonin (N-acetyl-5-c) has several putative functions, including regulation of circadian rhythms, regulation of the reproductive axis and antioxidant activity. Autoradiographic studies and receptor assays have demonstrated the presence of melatonin receptors in various regions of the central nervous system and in other tissues in humans.
Exogenous administration of melatonin has been found by several investigators to facilitate sleep onset and improve quality of sleep. Available data suggest that the sleep-inducing properties of melatonin may differ from those of benzodiazepines. Benzodiazepines decrease duration of REM sleep after single administration of a high dose or long-term administration of low dose. Benzodiazepines also reduce slow-wave sleep, thus negatively influencing sleep quality. In contrast, a single low dose of melatonin produced no suppression of REM sleep. Furthermore, unlike benzodiazepines, melatonin does not induce xe2x80x9changoverxe2x80x9d effects.
In a previous publication of one of the inventors, British Journal of Anesthesia 82(6):875-80(1999), low-level dosing of oral melatonin in a sublingual fashion was demonstrated as an effective pre-medication, prior to administering a general anesthetic. Patients who were administered such low-level doses sublingually had a significant decrease in anxiety levels and an increase in levels of sedation before operation. However, as pointed out in that article, the use of melatonin in anesthesia had as of then never been evaluated properly, and to the inventor""s present knowledge it has never been used as a general anesthetic prior to this series of applications.
The invention of Ser. No. 09/927,687 had as its primary objective the development of pineal hormone melatonin (N-acetyl-5-methoxytryptamine) or its biologically active analogues as a general anesthetic which can be used without any significant anesthetic hangover. The continuing need in the art for meeting that objective was readily apparent.
With reference to the continuing need referred to above, applicants have continued to work with melatonin and its analogues to derive improved compounds which may be used for anesthetic effect generally and in small doses for hypnotic effect sedation or even sleep inducement. This continuing work has evolved into the discovery that 2-trihalo methyl melatonins and in particular the 2-trifluoromethylmelatonin are substantially more active in anesthetic effect than melatonin itself. The result of this increased activity means that the compounds may be used in larger doses for general anesthesia, but in smaller doses for hypnotic effect and sedation and sleep effect.
Further discoveries since the filing of the original application have revealed a particularly effective pharmaceutical carrier for melatonin, melatonin analogues and the improved derivatives of the present invention. The carrier allows dissolving and high concentrations of melatonin or its analogues. The preferred carrier is comprised of one volume of 1-methyl-2-pyrrolidinone, one volume of propylene glycol and two volumes of water. It goes without saying that the volumetric ratios of these carrier solvents may be varied somewhat, depending upon the circumstances.
Anesthetic compositions are prepared using a pharmaceutically-acceptable carrier, preferably a preferred carrier comprising a mixture of one volume of 1-methyl-2-pyrrolidinone, one volume of propylene glycol and two volumes of water and an anesthetic-inducing effective amount of melatonin or biologically active analogues of melatonin such as 2-trifluoromethylmelatonin. The invention also relates to the method of administration using the described compositions.
N-acetyl-5-methoxytryptamine (melatonin) is synthesized mainly by the pineal gland, and to a lesser extent by extra pineal tissues such as the retina, harderian gland, and gastrointestinal tract. Melatonin has the following structure: 
As seen, the chemical formula for melatonin is N-acetyl-5-methoxytryptamine. From time to time in the specification applicant uses the term xe2x80x9cN-acetyl-5-methoxytryptamine (melatonin), or its biologically active analoguesxe2x80x9d. As used herein, this phrase refers to the precise compound itself and other compounds having the same general structure, but only differing in minor moieties, and therefore still having the same biological activity of anesthetic-inducing effectiveness. The biologically active compound of the present invention, such as melatonin, may be derived or extracted from the pineal gland, or it can be synthesized from 5-Methoxyindol as a starting material by known routes, Szmuszkovicz et al., J. Org. Chem. 25, 857 (1960). Biochemical role of melatonin: Chem. and Eng. News 45, 40 (May 1, 1967).
The analogue of the present invention that has been found to be more active than melatonin itself, and therefore can be used in smaller dosage levels and even at such small dosage levels to effectively induce hypnotic state, sedation or sleep, is 2-trifluoromethylmelatonin. As can be seen from a comparison with the formula for melatonin it contains a carbon trifluoromethyl moiety at the 2-position, replacing a hydrogen moiety from melatonin. While 2-trifluoromethyl-melatonin is the most effective so far found to date, it may be that other 2-position moieties such as 2-trihalo moieties in general can be used. Therefore, within the term 2-trihalo we intend to encompass chloride, fluoride, bromide and iodide. 
The anesthetic active, i.e., the N-acetyl-5-methoxytryptamine (melatonin), or its biologically active analogues, can be administered with traditionally acceptable pharmaceutical carriers as described in the patent applications. Examples include Intralipid(copyright), Cyclodextrin, and others, some of which are briefly hereinafter described. However, there is no need for detailed description of suitable anesthetic carriers because they are so well known in the industry. Here, however, the present applicants have discovered a preferred pharmaceutical carrier system.
Melatonin has previously been administered to animals in organic solvents that have central nervous system (CNS) effects. Such organic solvents frequently consist of ethanol in water. An administration vehicle not having CNS effects is desired for the administration of melatonin to achieve pure melatonin effects.
It was discovered that melatonin could be dissolved in high concentrations in a solvent comprised of 1 volume 1-methyl-2-pyrrolidinone, 1 volume propylene glycol and 2 volumes of water. Melatonin can be dissolved up to a concentration of 300 mg/ml in this solvent. The volume ratios here expressed are preferred but generally can be within the range of 25% or less by volume of 1-methyl-2-pyrrolidinone.
Intravenous administration of melatonin in this solvent system results in a rapid increase in blood melatonin concentrations in rats that are suitable to cause an unexpected anesthetic effect without causing toxic side effects.
Formulations containing melatonin analogues that consist of melatonin or its analogues and 1-methyl-2-pyrrolidinone in water can be used or formulations containing melatonin analogues and 1-methyl-2-pyrrolidinone combined with water and other known inert solvents such as propylene glycol, polypropylglycol, polysorbitans and cyclodextrins can be used.
Derivatives or analogues of melatonin, such as 2-bromomelatonin and 2-phenylmelatonin may be administered in solvents described above containing 1-methyl-2-pyrrolidinone for delivery to mammals.
As earlier expressed, 1-methyl-2-pyrrolidinone may be present in the disclosed vehicles at concentrations less than 25% volume/volume. For example, concentrations of 1-methyl-pyrrolidinone may range from 5 to 25% or greater than 25% in water, or in water combined with propylene glycol, glycerol, dextrins and/or polysorbitans.
The composition may be administered by conventional administration methods for anesthetics, i.e., oral administration, nasal respiratory administration, bolus injection, intravenous administration by repeated doses or by continuous infusion, rectal, vaginal, sublingual, cutaneous and slow release routes. It may be, and often is preferred, that it be administered in two or more ways, such as by bolus injection followed by continuous intravenous administration.
The liquid forms in which the novel compositions of the present invention may be incorporated for administration orally or by injection include aqueous solutions, suitably flavored syrups, aqueous or oil suspensions, and flavored emulsions with edible oils such as cottonseed oil, sesame oil, coconut oil or peanut oil, as well as elixirs and similar pharmaceutical vehicles. Suitable dispersing or suspending agents for aqueous suspensions include synthetic and natural gums such as tragacanth, acacia, alginate, dextran, sodium carboxymethylcellulose, methylcellulose, polyvinylpyrrolidone or gelatin.
Preferred compositions for administration by injection include those comprising a melatonin biologically active analogue as the active ingredient, in association with a surface-active agent (or wetting agent or surfactant) or in the form of an emulsion (as a water-in-oil or oil-in-water emulsion).
Suitable surface-active agents include, in particular, nonionic agents, such as polyoxyethylenesorbitans (e.g. Tween(trademark) 20, 40, 60, 80 or 85) and other sorbitans (e.g. Span(trademark)20, 40, 60, 80 or 85). Compositions with a surface-active agent will conveniently comprise between 0.05 and 5% surface-active agent, and preferably between 0.1 and 2.5%. It will be appreciated that other ingredients may be added, for example mannitol or other pharmaceutically acceptable vehicles, if necessary.
Suitable emulsions may be prepared using commercially available fat emulsions, such as Intralipid(trademark), Liposyn(trademark), Infonutrol(trademark), Lipofundin(trademark) and Lipiphysan(trademark). The active ingredient may be either dissolved in a pre-mixed emulsion composition, or alternatively it may be dissolved in an oil (e.g. soybean oil, safflower oil, cottonseed oil, sesame oil, corn oil or almond oil) and an emulsion formed upon mixing with a phospholipid (e.g. egg phospholipids, soybean phospholipids or soybean lecithin) and water. It will be appreciated that other ingredients may be added, for example glycerol or glucose, to adjust the tonicity of the emulsion. Suitable emulsions will typically contain up to 20% oil, for example, between 5 and 20%. The fat emulsion will preferably comprise fat droplets between 0.1 and 1.0 xcexcm, particularly 0.1 and 0.5 xcexcm, and have a pH in the range of 5.5 to 8.0.
Particularly preferred emulsion compositions are those prepared by mixing an active compound with Intralipid(trademark) or the components thereof (soybean oil, egg phospholipids, glycerol and water).
Compositions for inhalation or insufflation include solutions and suspensions in pharmaceutically acceptable, aqueous or organic solvents, or mixtures thereof, and powders. The liquid compositions may contain suitable pharmaceutically acceptable excipients as set out above. Preferably the compositions are administered by the oral or nasal respiratory route for local or systemic effect. Compositions in preferably sterile pharmaceutically acceptable solvents may be nebulised by use of inert gases. Nebulised solutions may be breathed directly from the nebulising device, or the nebulising device may be attached to a face mask, tent or intermittent positive pressure breathing machine. Solution, suspension or powder compositions may be administered, preferably orally or nasally, from devices which deliver the formulation in an appropriate manner.
The anesthetic may be used alone or often in combination with other anesthetics simultaneously administered. Put another way, it will be appreciated that when using any combination described herein, both the compound of melatonin or its analogue and the other active agent(s) can be administered to a patient, within a reasonable period of time. It may indeed act synergistically with other anesthetic drugs. The compounds may be in the same pharmaceutically acceptable carrier and therefore administered simultaneously. They may be in separate pharmaceutical carriers such as conventional oral dosage forms which are taken simultaneously. The term xe2x80x9ccombinationxe2x80x9d also refers to the case where the compounds are provided in separate dosage forms and are administered sequentially. Therefore, by way of example, one active compound may be administered as a tablet and then, within a reasonable period of time, the second active component may be administered either as an oral dosage form such as a tablet or a fast-dissolving oral dosage form. By a xe2x80x9cfast dissolving oral formulationxe2x80x9d is meant, an oral delivery form which, when placed on the tongue of a patient, dissolves within about 10 seconds.
The dosage will vary depending upon the deepness of the anesthesia desired, but based upon limited studies to date, it is believed that the dosage most effective will be within the range of 0.001 mg/kg of body weight to about 500 mg/kg of body weight, more predictably preferred is the range of 5 mg/kg of body weight to about 350 mg/kg of body weight.
The synthesis of 2-trifluoromethylmelatonin may be summarized by the following reaction scheme. 
In word description, the reaction synthesis of 2-trifluoromethylmelatonin can be described as follows. 2-Iodomelatonin (1 g, FW=232) was dissolved in 20 ml DMF in a round bottom glass reaction flask fitted with a condenser. CuI (1.3 g, FW=190) and methyl 2,2-difluoro-2-(fluorosulfonyl)acetate, 1.2 g, (FW=192) was added and the reaction mixture heated to 60-80xc2x0 C. at least 3 hr.
Following the reaction period, the reaction mixture was chilled on ice and DMF removed from the mixture by rotatory evaporation. 15 ml water was added and the reaction mixture neutralized. Melatonin products were extracted from the aqueous phase with methylene chloride (20 mlxc3x972). The products of the reaction were analyzed by GC/MS and found to contain a fraction that corresponds to trifluoromethylmelatonin (M+, m/z 300).
Trifluoromethylmelatonin analysis was performed by silica gel TLC (Silica Gel 60, Fisher Scientific, Inc.) using anhydrous ethyl acetate as mobile phase. Bands were detected by fluorescence (366 nm). Isolation of one band yielded pure trifluoromethylmelatonin. Recovery and weighing of the fraction demonstrates a yield of greater than 25-30%. 2-Trifluoromethylmelatonin was confirmed by proton and fluorine NMR. When patients are administered N-acetyl-5-methoxytryptamine (melatonin) or its biologically active analogues, there is a noticeable decrease in anesthetic hangover. It is believed that this occurs because melatonin itself is a naturally-occurring hormone synthesized in the body by the pineal gland.
The following anesthetic examples are offered to further illustrate, but not limit the invention disclosed herein.